Lithium ion secondary batteries provided with flammable non-aqueous electrolyte solutions and a large number of components for safety measures have a problem that the energy density per volume as a whole battery is small. Lithium ion secondary batteries provided with non-flammable aqueous electrolyte solutions that do not require the components of non-aqueous electrolyte solutions can provide a larger energy density per volume large. However, the potential windows of conventional aqueous electrolyte solutions are narrow. Therefore it has been considered difficult to obtain a high voltage from batteries having conventional aqueous electrolyte solutions.
To solve the above problem of aqueous electrolyte solutions, Non-Patent Literature 1 discloses expanding the potential window of an aqueous electrolyte solution by dissolving lithium bis(trifluoromethanesulfonyl)imide (hereinafter may be referred to as “LiTFSI”) in the aqueous electrolyte solution at high concentration. In Non-Patent Literature 1, a lithium ion secondary battery is formed by the combination of such a high concentration aqueous electrolyte solution, LiMn2O4 as a cathode active material, and Mo6S8 as an anode active material.
Non-Patent Literature 1: Liumin Suo, et al., “Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries, Science 350, 938 (2015)